Pea taxonomy is complex and debatable. In particular, there is no authoritative and definitive way to classify arvense (field) and hortense (garden) peas. They used to be considered as separate species (Pisum arvense, Pisum hortense) but they are now seen as separate varieties or subspecies of Pisum sativum (Pisum sativum var. arvense, Pisum sativum var. hortense, Pisum sativum subsp. hortense) or as separate varieties of the subspecies Pisum sativum subsp. sativum (Pisum sativum subsp. sativum var. arvense, Pisum sativum subsp. sativum var. sativum) (Martin-Sanz et al., 2011; USDA, 2011).

Peas (usually Pisum sativum L.) are one of the four most important legume crops next to soybean, groundnut, and beans. It is a particularly important legume grain in temperate areas with numerous food (dry seed, vegetable) and feed (seed, fodder) usages (Muehlbauer et al., 1997).

Peas are a fast-growing herbaceous legume with angular or roundish hollow stems covered with a waxy bloom. The plant has a tap root that can grow as deep as 1 m, with numerous lateral roots. Leaves are alternate, compound with 1-3 pairs of leaflets borne on petioles with several pairs of tendrils. Large (up to 10 cm long) leaf-like stipules are inserted at the base of the leaves (FAO, 2011; Muehlbauer et al., 1997; Oelke et al., 1991). The inflorescence is a raceme that bears white, pink or purple flowers. The colour of the flower is an indicator of tannin content in the seed: white-flowered peas produce tannin-free seeds while the seeds of coloured-flowered peas contain tannins (Prolea, 2008). Pods are dehiscent and contain several seeds that may be globose or angled, smooth or wrinkled (FAO, 2011; Muehlbauer et al., 1997).

Pisum sativum has a large genetic diversity and breeding programmes have generated many varieties. There are winter and spring varieties, leafy and leafless, early- or late-maturing, etc. Seeds can be green, yellow or pale green, brown or mottled, thin- or thick-hulled, smooth or wrinkled, of varying shape and size. Some varieties are tannin-free and pea breeding offers a large opportunity to improve the nutritional characteristics of the grain (Hickling, 2003; Gatel et al., 1990; Canbolat et al., 2007; Kosev et al., 2010).

Garden peas (fresh peas, green peas, vining peas) are harvested while still immature to be eaten cooked as a vegetable. They are marketed fresh, canned, or frozen. Garden peas are usually of the white-flower hortense types.

Field peas (dried peas, combining peas) are harvested ripe. Dried peas are used whole, split, either made into flour for human food (Muehlbauer et al., 1997) or fed to livestock. Field peas are usually from the coloured flower arvense type. However, "protein peas" (pois protéagineux in French) are field peas that have been developed in Europe from hortense (and hybrid) varieties in the 1980s as a high-protein, white-flowered, low-tannin, low-antitrypsin protein source for animal feeding (GNIS, 2011). Field peas used for animal feeding (feed peas) can be fed raw or processed in order to improve their nutritional value.

Pea crops grown primarily as a forage crop. An example of such varieties is the Austrian Winter pea. See the Pea forage datasheet for more information.

It should be noted that vernacular or trade classifications of peas are often tied to specific agricultural systems and feed or food applications. Pea varieties can be multi-purpose: food and feed, feed and forage. The crop residues can be fed to livestock and pea forage can also be used for green manure (Maxted et al., 2001; Oelke et al., 1991).

Distribution

Peas were one of the first cultivated crops, around 7000-6000 BC, and are thought to have originated from South-Western Asia (possibly North-Western India, Pakistan, Afghanistan and Central Asia). Peas would then have spread westwards to Russia, Europe and the Mediterranean Basin but also eastwards to China (Chittaranjan, 2007; Oelke et al., 1991). Production then spread to the Western Hemisphere upon discovery of the New World. Peas are also cultivated in Africa as a winter crop (Messiaen et al., 2006).

Peas occur in a wide range of environments. They can be grown up to an altitude of 1000 m in equatorial areas (and up to 1800-3000 m in Ethiopia) (Messiaen et al., 2006). Peas grow better in relatively cool climates with average temperatures between 7 and 24°C, and in areas with 800-1000 mm annual rainfall mostly distributed during early stages of growth (Messiaen et al., 2006; Oelke et al., 1991). They can be found on a wide range of soils from sandy loams to heavy clays provided the soil is well-drained. Ideal soil pH is 5.5-6.5, but a pH of 7-7.5 may not hamper growth if the soil is not overlimed and prone to manganese deficiency (FAO, 2011; Oelke et al., 1991). Acidic soils, high aluminium soils and waterlogged areas are deleterious to pea growth (Messiaen et al., 2006). Hot weather and drought stress are particularly damaging to peas during the flowering period. Field peas can be grown as a winter crop in warm and temperate areas because pea seedlings have considerable frost resistance. Where winters are too cold, peas can be grown as a spring crop. They only require 60 days to reach the bloom stage and 100 days to mature and dry (Oelke et al., 1991).

Processes

Pea harvest

Pea crops yield pea seeds and pea straw that can be used as forage: each hectare supplies about 1.7 t of seeds and 2-3 t of pea straw (Prolea, 2008).

Seeds processing

Peas are a good source of protein and energy but their feed value is highly variable and depends on several factors, particularly anti-nutritional factors (trypsin inhibitors and lectins), protein genicity, protein structure and fibre content. Many processes have been developed to improve the nutritive value of peas: mechanical treatments (grinding and decortication), dry or wet heat treatments (cooking and autoclaving) and their combinations (flaking, extrusion, pelleting, etc.). The processes must be tailored to the nutritional requirements of each livestock species.

Grinding

Grinding breaks cell walls, allowing cell contents to come in contact with digestive enzymes. Starch escapes from the cell walls after coarse or moderate grinding (between 160 µm and 800 µm) while protein requires finer grinding (down to 30 µm) to be released (Perrot, 1995). For ruminants, who masticate more efficiently than monogastric animals, coarse grinding is sufficient (Corbett, 1997).

Decortication/dehulling

The hull of the pea represents 7-14% of seed weight and consists primarily of cellulose and xylan (Castell et al., 1996). Decortication (dehulling) removes about 10% of the seed fibre, thus increasing the proportions of protein and starch, and reducing the amount of poorly digestible or indigestible fibre. In coloured peas, removing the hulls has the additional advantage of decreasing the tannin content, which also increases the nutritive value of the seeds (Perrot, 1995).

Air classification

Pea flour can be air-classified to extract the denser protein fraction, yielding a pea protein concentrate. See the Pea protein concentrate datasheet for more detail.

Heat, water and pressure treatments

Heat, water and pressure treatments can be divided into 3 categories: dry heat (roasting, micronising), wet heat (steam flaking, autoclaving) and cooking-extruding (Poncet et al., 2003). Their application to pea seeds has two main objectives depending on the target animal species.

In monogastric animals, the goal is usually to reduce antinutritional factors. Trypsin inhibitors have a compact structure that can be resistant to enzymes and to moderate heat processing such as steam pelleting at 80°C. Harsher conditions, such as autoclaving at 100°C for 15 minutes or cooking-extrusion are necessary to inactivate them. Lectins also require relatively high temperatures (Perrot, 1995).

Heat treatments and combinations of heat, water and pressure are also effective at modifying protein structure and at decreasing their resistance to enzymes (Poncet et al., 2003). In monogastrics, this can be beneficial, but the harsh conditions required (more than 100°C) for the deactivation of antinutritional factors tend to decrease protein solubility and result in other unwanted side effects, such as the Maillard reaction, which binds sugars and amino-acids (Perrot, 1995). In ruminants, heat treatments have the desirable effect of reducing protein degradability, resulting in increasing rumen by-pass protein and thus N utilization in the intestine (Rémond et al., 1997). Rumen bypass protein can be doubled by wet heat processing between 105 and 136°C for 10-15 min (Poncet et al., 2003). However, heat treatments can also increase starch degradability in the rumen, which is not desirable (Corbett, 1997).

Environmental impact

Agronomic benefits

Peas are much valued in rotations with cereals because their cultivation breaks cereal disease cycles, facilitates weed control and improves soil condition and fertility (Chittaranjan, 2007).

N-fixing legume

Pea crops greatly improves soil fertility status. Peas are N-fixing legumes that may require seed inoculation with Rhizobium leguminosarum in order to enhance nodulation. Inoculation becomes necessary when soil pH is below 5.7. Pea crops decrease fertilizer requirements of the following crops by 30-50 kg/ha (Muehlbauer et al., 1997). On a 3-year rotation including peas, average N savings were about 140 kg N/ha. However, it is important that the crops following peas undergo succession and cropping practices that limit nitrate losses (Charles et al., 2001).

Nutritional aspects

Nutritional attributes

Pea seeds are a protein and energy feed. Peas are regarded as a highly valuable protein source for animal nutrition due to their high protein content (usually about 22-24%, ranging between 16-32% DM), which is intermediate between cereals and oil meals (Castell et al., 1996; Feedipedia, 2011). The protein content tends to be higher for winter peas than for spring peas, and lower in smooth seeds than in wrinkled ones (Gatel et al., 1990).

Peas can be an alternative to soybean meal. Their amino-acid profile is well-balanced in lysine (similar to that of soybean meal and higher than those of cereal grains, particularly maize grain) so that they can be a protein supplement in cereal-based diets (Duranti et al., 1997). However, they are deficient in tryptophan and sulphur-containing amino-acids (notably methionine) for species where these are essential amino-acids (Dixon et al., 1992; Gatel, 1995; Perrot, 1995; Vander Pol et al., 2008). Peas can be a particularly valuable protein source in organic livestock farming when usual sources such as soybean meal and industrial amino-acids are prohibited, for instance due to concerns regarding GMOs (Schumacher et al., 2011; Martini et al., 2008).

It should be noted that given the extent of different cultivars grown and seed types produced, there is a considerable range in the composition and, consequently, nutritive value of peas (Castell et al., 1996).

Peas also have a high starch content (48-54% DM) though some varieties, such as the marrowfat peas used in the United Kingdom, can have less than 45% DM as starch (Feedipedia, 2011). Spring peas and smooth peas contain more starch than winter and wrinkled peas respectively (Gatel et al., 1990). Pea seeds are relatively low in fibre (NDF 10-18% DM; ADF 6-8% DM; ADL less than 0.5% DM) and oil (less than 1.5% DM) (Feedipedia, 2011).

As described in Potential constraints below, contents in tannins and trypsin inhibitors depend on the variety. Protein peas have particularly low levels of antitryptic activity (2-6 TIU/mg), very low levels of tannins and a higher protein content (23-24% DM) (GNIS, 2011).

Pea seeds are an excellent binder for pelleting or cubing (Anderson et al., 2002). They have a positive effect on pellet quality, with a index of 6 (on a 10 scale according to Thomas et al., 2001), higher than the indices of 5, 5 and 4 for barley, maize and soybean meal respectively.

Potential constraints

Antitryptic activity

Trypsin inhibitors are the main antinutritional factor in peas, although peas are one of the grain legumes with the least trypsin inhibitor content, usually lower than 2% of the protein content (raw soybeans contain 8 times this amount). There are large varietal differences in antitryptic factors. The trypsin inhibiting activity of 33 European spring pea varieties ranged from 1.69 to 7.56 trypsin inhibiting units (TIU), while the level in winter peas was 7.34-11.24 TIU (Leterme et al., 1998). Smooth peas contain more trypsin inhibitors than wrinkled peas (Leterme et al., 1989; Perrot, 1995).

Trypsin inhibitors bind with trypsin in the small intestine, preventing protein digestion. They also induce pancreatic enlargement and increase protein secretion, causing lower N retention, lower growth and lower feed efficiency in monogastric species, including pre-ruminant animals (Perrot, 1995; Rackis et al., 1986). In ruminants, trypsin inhibitors are degraded in the rumen and are not a concern (Fuller, 2004).

Tannins

Tannins are known to reduce protein digestibility in monogastrics because they bind with protein prior to their digestion. Tannin content is related to the seed color, grains with dark seed coats containing more tannins (Myer et al., 2001). Tannin content is also much lower in white flowered peas than in coloured flowered peas (Grosjean et al., 1986; Canbolat et al., 2007; Prolea, 2008).

Lectins

Lectins are proteins able to bind glycoproteins and carbohydrates. They act in the small intestine by interfering in the absorption of the end-products of digestion by binding and disrupting the epithelial cells (Dixon et al., 1992). They represent about 2.5% of pea protein (Perrot, 1995).

Improving the nutritive value of peas by decreasing trypsin inhibitors and tannins is the goal of many breeding programs (Gatel, 1995). Modern cultivars of "protein peas" are tannin-free and have low concentrations of trypsin inhibitors, which makes them particularly suitable for animal feeding, even in the unprocessed form for monogastrics (Mihailovic et al., 2005).

Ruminants

Pea seeds are a dual purpose ingredient. They provide half as much as crude protein as soybean meal with a lower rumen undegradable protein content, with the high energy value of a starch-rich feed (Schroeder, 2002). It represents one of the best quality and least expensive feeds, its low cost partly reflecting the low cost of transportation (Anderson et al., 2002).

Peas have a low starch degradation rate (from 4 to 6%/hour) that is much slower than that of cereals, such as barley (21 to 34%/h). This low rate decreases the risk of acidosis with high starch diets, as the pH lowest point is higher than that observed with barley when fed at a level of 70% with oaten hay (Valentine et al., 1987).

Palatability

Nitrogen value

The in sacco nitrogen degradability of peas is very high. This is due to the large amount of highly soluble proteins but it is also an artifact of the in sacco method: grinding peas produces many very fine particles that pass through the pores of the nylon bag and are considered to be fermented in the rumen, which is not the case (Maaroufi, 2001). When peas are included in a diet with a limited fermentable organic matter, the fraction washed out from the bag is supposed to be lost and rejected as urinary nitrogen, which leads to an underestimation of the amount of protein available in the intestine (Cabon et al., 1997).

Nevertheless, the ammonia concentration in the rumen increases with the substitution of soybean meal and maize grain by peas (Vander Pol et al., 2009). Peas are an acceptable source of protein when properly balanced in the diet with by-pass protein sources. The nitrogen of peas is highly soluble (40%) (Aguilera et al., 1992). Grain processing methods substantially affect solubilisation and fermentation (Azarfar et al., 2007). The partition between the soluble and degradable fractions for both DM and N varies a lot between cultivars, for example from 58% to 85% for soluble N (Cabon et al., 1995).

Effects of processing

Grinding

Increasing the fineness of grinding peas (from 6 mm to 0.8 mm sieve) increased their N degradability in the rumen by 12.3 percentage points (Michalet-Doreau et al., 1991). When grinding pea seeds, the chemical and physical characteristics of the differently sized fractions reflect the respective comminution laws of the hulls and the kernels, leading to a physical separation of the botanical constituents of the pea seed, with coarse fractions containing hulls and most of the parietal constituents, finer fractions containing kernels and cellular constituents, and the finest fraction being mainly composed of starch granules (Maaroufi et al., 2000a). These different fractions have quite different fermentation patterns (Maaroufi et al., 2009). Grinding pea seeds increases their fermentability, resulting in a decrease in the post-prandial in vivo pH (Giger-Reverdin et al., 2000) due to a higher concentration in soluble carbohydrates (Maaroufi et al., 2000b).

Heat treatment

Autoclaving peas decreased both the soluble fraction and the fractional rate of protein degradation of the slowly degraded fractions, strongly reducing effective N degradability (Aguilera et al., 1992; Goelema et al., 1999). Toasting decreased both total protein digestibility and intestinal digestibility of rumen undegraded protein. These results can also be explained by the particle size reduction due to processing. However, toasting does not affect rumen undegraded starch or total starch digestibility (Goelema et al., 1999).

Pelleting

Pelleting increased the washable protein fraction and the fractional degradation rate, thus decreasing the amount of rumen undegraded protein. This is due to the particle size reduction occurring during processing (Goelema et al., 1999).

Young dairy cattle

Peas can be included at up to 40 to 50% DM in concentrates fed to pre-weaned and weaned dairy calves. It can partly replace maize grain, barley and/or soybean meal (Schroeder, 2002). Peas may be the sole protein source for dairy heifers (Anderson et al., 2002).

Dairy cows

Peas are palatable to dairy cows (Weiss, 1992). Replacing barley by peas increased DM intake and subsequently the yields of milk, fat and protein (Valentine et al., 1987). Peas can be included at up to 25% in concentrates for lactating cows (Anderson et al., 2002). Peas have been successfully substituted for soybean meal in cows producing up to 30 kg milk/d (Hoden et al., 1992), for soybean/canola meal (Corbett et al., 1995) and for soybean meal and barley in late lactation (Khorasani et al., 2001). At higher levels of production, they should be supplemented with better protein sources, especially some with higher concentrations of sulphur-containing amino acids. The substitution of soybean meal and maize grain with field peas in dairy cow diets at a 15% inclusion rate does not modify the organoleptic characteristics of milk (Vander Pol et al., 2008).

Peas have to be coarsely ground for dairy cow diets to avoid depression in total tract digestibility of nutrients (Vander Pol et al., 2009). Raw and extruded peas fed at 20% of the diet to cows in early lactation resulted in similar DM intake, milk yield and milk composition, though extrusion increased the rumen degradability of starch. Milk yield and composition were also similar to that of the soybean meal-based control diet (Petit et al., 1997).

Beef cattle

As peas are very palatable, they are best used in diets where nutrient density and palatability are important, such as creep feeds where its optimum inclusion rate lies between 33 and 67% (Anderson et al., 2002). Peas can replace a mixture of barley and canola meal for young calves (Schroeder, 2002). They can also be used as an ingredient in creep feed to increase calf weight gain without impairing rumen fermentation and digestion (Gelvin et al., 2004).

For growing steers and heifers, peas should not constitute more than 25% of the total diet to prevent excessive protein intake (Anderson et al., 2002). It is a suitable supplement to maize silage for growing cattle (Gatel, 1995). At a level of 20% in a total mixed ration fed ad libitum, replacement of a maize grain and rapeseed meal diet by field peas increased DM intake without impairing fermentation characteristics: pH and NH3 concentration remained unchanged, and total volatile fatty acids and acetate concentration decreased. Nevertheless, average daily gain increased due to the increase in DMI (Gilbery et al., 2007). These results indicate that field peas can be included successfully into rations at levels up to 36% (diet DM) without negatively affecting growth and most carcass characteristics of finishing beef cattle (though effects on marbling score were variable). These data also indicate that the energy content of field peas is similar to that of cereal grains, such as maize and barley, when included in high-concentrate finishing diets (Lardy et al., 2009).

Feeding peas to steers results in growth and carcass characteristics similar to that obtained with dry-rolled maize, and improves objective and subjective tenderness, overall desirability and flavour of beef. Field peas could be fed to cattle and give positive attributes to the quality of the meat up to 30% inclusion in the diet (Jenkins et al., 2011).

Sheep

For finishing lambs, peas have a similar energy value to maize (Loe et al., 2004). They were included at a level of 45% in a feedlot diet and replaced all soybean meal and part of the maize. Growth, carcass fatness, meat pH, colour parameters and cooking losses were comparable among groups (Lardy et al., 2002). The replacement of soybean meal with peas did not significantly affect growth and slaughter performances, and preserved meat quality (Lanza et al., 2003). The use of pea seeds increases the proportions of total n-3 fatty acids, and meat from lambs fed peas showed a more favourable n-6:n-3 ratio in the intramuscular fatty acid composition (Scerra et al., 2011).

Goats

Dairy goats in mid-lactation cope well with a total mixed ration including field peas at a level of 23% even though the values for protein digestible in the intestine seem to be underestimated (Maaroufi, 2001).

Pigs

Peas are a good source of energy and protein for pigs, and as such is the grain legume most used for pig feeding in Europe and Canada. Because of the large variability in nutrient composition and anti-nutritional factors in peas, it is recommended to characterize the raw material as accurately as possible (origin, variety, chemical composition, anti-nutritional factors) before using it in pig feeding.

For pigs, peas are a suitable source of lysine and threonine but, like other grain legumes, it is deficient in sulfur-containing amino acids when its profile is compared to the ideal amino acid balance for pigs. In addition, the standardized ileal digestibility of amino acids is lower for peas than for soybean meal (Noblet et al., 2002), due to the presence of anti-nutritional factors (see Potential constraints above) (Grosjean et al., 2000). The use of pea cultivars with high antitryptic activity should be limited in pigs, but processing methods such as extrusion can have positive effects on protein and amino acid digestibility (O'Doherty et al., 2001; Stein et al., 2007).

According to the review of Gatel et al., 1990, the energy digestibility and digestible energy content of peas varies from 76 to 92% and from 14.2 to 17.2 MJ/kg DM, respectively, for growing pigs. This variability seems to be reduced in sows (Premkumar et al., 2008).

In general, ground raw peas are palatable and can be fed to all classes of pigs with few problems, except in young pigs (under 20 kg BW) (Myer et al., 2001). Starter diets can contain up to 10% ground field peas and processing (extrusion, toasting, steam pelleting) the peas could increase the maximum recommended level up to 20% (Gatel et al., 1990; Myer et al., 1993). Above this rate, growth performances are generally reduced (Gatel et al., 1990). This effect is mainly explained by a low palatability of the diet, an imbalance in secondary limiting amino acids (methionine, tryptophan) and a low digestibility or availability of amino acids (Friesen et al., 2006). For growing-finishing pigs, ground raw peas could be included as the only source of supplemental protein in the diet provided that the amino acid balance is optimal (methionine or methionine + tryptophan) (Bastianelli et al., 1995; Vieira et al., 2003). It has been suggested that a mixture of peas and rapeseed meal was a better supplement for growing-finishing pigs than peas alone since rapeseed meal is rich in sulfur-containing amino acids while peas are a superior source of lysine (Castell et al., 1993).

Data on inclusion of raw peas in gestation and lactation diets are scarce. Spring peas used at 24% in gestation and lactation diets did not reduce reproductive performance (Gatel et al., 1988).

Poultry

Peas are a good source of protein and energy for poultry. However, they are deficient in sulfur-containing amino-acids and, while feed pea varieties containing low concentrations of anti-nutritional factors have been available for several decades, there is still a lot of variability in composition and nutritional factors, resulting in differences in digestibility (Nalle et al., 2011). When the nutritional value of field peas (raw or processed) is well defined, and when it is possible to balance the diet with synthetic amino acids, they can be included at high levels in broiler diets without negative effects on performance. Maximum recommended levels range from 20% (Nalle et al., 2011) to 30-35% (Farrell et al., 1999; Diaz et al., 2006) for broilers. For laying hens, similar inclusion rates are suggested (Perez-Maldonado et al., 1999). As a consequence, in industrial farming conditions, field peas can be used without maximum level (Lessire, unpublished).

Processing may improve starch digestibility, and heat treatments may alleviate the negative effects of some antinutritional factors. Among those treatments, pelleting, extrusion and cooking are frequently mentioned. Peas can be treated alone or mixed with other raw materials such as full fat oilseeds. Many experiments have been performed to assess individual pea varieties or to measure the effect of processing methods on metabolizable energy (ME) value, digestibility, poultry growth, laying rate and feed efficiency. For example, Grosjean et al., 1999 reported a large variation in nutritional value of peas determined on adult cockerels, and did not find a strong correlation with chemical composition. Peas with low ME values benefited the most from pelleting, which greatly increased their ME, starch digestibility and protein digestibility. After pelleting, starch digestibility (more than 98%) was found to be not far from that observed for cereal grains, though starch digestibility was lower for wrinkled peas (84%).

Rabbits

Field peas could be introduced without any problem at up to 30% in pelleted feeds for growing rabbits and completely replace soybean meal as the main protein source, despite their lower protein content (Colin et al., 1976; Franck et al., 1978; Seroux, 1984). Peas can replace not only soybean meal but also a part of the cereal grains in the diet. In some experiments, growth performance improved when peas replaced the equivalent proportion of soybean meal + cereal grain (Castellini et al., 1991). When a higher proportions of peas are included in complete feeds, it becomes difficult to maintain the nutritive balance of all nutrients; for example a significant reduction of growth rate was observed in 2 trials out of 3 (-10% on average) when 45% of peas replaced soybean meal + cereal grains (Franck et al., 1978). However, a diet containing 60% peas was considered to be fairly acceptable for rabbit feeding from weaning (28 d) to slaughter age (75 d) even for the youngest rabbits, despite the relatively high starch level in peas (45%) (Gidenne et al., 1993). In addition, good growth performance was obtained with a concentrate diet containing 67.7% peas distributed together with 150 g/d of alfalfa hay. These authors also demonstrated that toasting peas failed to provide any significant advantage over raw peas (Johnston et al., 1989). Field peas varieties with high or low levels of antitryptic activity (12.7 vs. 2.7 TIU/mg DM) resulted in identical growth performance and feed efficiency (Seroux, 1984).

In experiments with breeding does, field peas replaced soybean meal and cereal grains in the control diet at 21% for 16 months (Seroux, 1988) and 40% for 4 months (Franck et al., 1978). In both cases reproduction was similar to that of the control diet with only some small positive or negative effects, which were usually non significant. It was concluded that a long term inclusion of 21% field peas is suitable for feeding breeding does.

In addition to these nutritional considerations, the positive effect of field peas on pellet quality, which is slightly higher than that of barley, maize and soybean meal, must be underlined (see Nutritional attributes above) (Thomas et al., 2001). This is important considering that peas frequently replace these ingredients in rabbit trials.

Fish

The high protein and starch content of peas make them a suitable replacement for traditional protein and energy sources in fish farming. Peas have been successfully tested in many fish species, as shown below. The main limitation of using peas in fish is the content in anti-nutritional factors (particularly trypsin inhibitors) and the fibre content. Grinding, heating and dehulling may improve the nutritive value of peas, depending on the fish species.

Carp (Cyprinus carpio)

In carp fed 40-45% raw, autoclaved or dry-cooked peas (low trypsin inhibitor varieties), replacing maize starch and soybean meal, final weight, daily gain and digestibility were found to be similar for heat-processed peas and the control diet, and higher than those obtained with raw peas. Dry cooking proved to be slightly more effective than the moist heat treatment in terms of growth performance, feed utilization efficiency and apparent digestibility in common carp (Davies et al., 2010).

Milkfish (Chanos chanos)

Finely ground peas are an acceptable protein source and can replace up to 20% of the total dietary protein in milkfish diets (Borlongan et al., 2003).

Rainbow trout (Oncorhynchus mykiss)

The recommended inclusion rate of peas in rainbow trout diets is about 20%. The effects of technology have been discussed by several authors. Dehulling, autoclaving, extrusion, and combinations of these processes have generally positive (though variable) effects. The protein digestibility of peas is high with observed values ranging from 88% for extruded peas (Burel et al., 2000) to 91-95% for raw-whole, raw-dehulled, and extruded-dehulled peas (Thiessen et al., 2003). Starch digestibility of raw peas (whole or dehulled) was found to be very low (14-25%; Thiessen et al., 2003) while starch digestibility in extruded peas was much higher, with values ranging from 69% (Burel et al., 2000) to 100% (Thiessen et al., 2003). Raw or cooked-extruded peas could be included at 20% in the diet of rainbow trout, replacing fish meal, resulting in higher growth than for the control diets, and no significant differences in protein and lipid digestibility, and in carcass composition. Heat treatment slightly improved the nutritional value of pea meal (Gouveia et al., 1993). Dehulled peas were also found to be suitable ingredients at 20% and autoclaving or extrusion increased starch digestibility, thus increasing energy and dry matter digestibility of whole-dehulled peas (Thiessen et al., 2003). Dehulled peas were found to have a higher DM and energy digestibility in rainbow trout (Oncorhynchus mykiss) after autoclaving for 5 minutes (Hernandez et al., 2010).

A co-extruded mixture of peas and rapeseed meal was used in rainbow trout diets at levels of up to 20% without negative effects on growth, nitrogen or energy utilization and muscle fatty acid composition (Gomes et al., 1993).

European sea bass (Dicentrarchus labrax)

Up to 40% dry-cooked and milled pea seed inclusion was feasible in diets of juvenile sea bass, allowing for a 12% reduction in fish meal content and a 25% substitution of carbohydrate content without appreciable loss in growth performance (final weight and growth rate) or diet utilization. However, carbohydrate digestibility was markedly affected, contributing to a significant reduction in DE (Gouveia et al., 1998). In a later experiment, an extruded, dehulled and microground pea seed meal included at 30% in the diet of juvenile sea bass proved to be an excellent supplementary ingredient that did not appreciably affect digestibility coefficients for protein, lipid and carbohydrates, and overall carcass composition (Gouveia et al., 2000).

Gilthead sea bream (Sparus aurata)

Pea seed meal (extruded, dehulled and microground, or treated by infrared radiation and ground) may replace up to 20% fish meal protein in diets for gilthead sea bream juveniles without affecting fish performance (Pereira et al., 2002). Whole extruded peas were used in farmed sea bream diets up to 35% without negative effects, replacing other carbohydrate sources and part of the fish meal (Adamidou et al., 2011).

Australian silver perch (Bidyanus bidyanus)

Ground field peas had a high protein digestibility (81%) in Australian silver perch but rather low DM and energy digestibility (51%) (Allan et al., 2000). Digestibilities increased significantly after dehulling (88%, 64% and 63% for protein, DM and energy digestibility, respectively) (Booth et al., 2001). In silver perches fed peas at 30-50%, dehulling and extrusion greatly improved OM, DM and energy digestibility. Although higher overall, digestibilities declined with increasing content of the extruded peas while there were only minor effects from inclusion of raw peas. Crude protein digestibility was lower than that of soybean meal but higher than for rapeseed meal (Allan et al., 2004).

Tilapia (Oreochromis niloticus)

The digestibilities in tilapia of DM, protein and energy of extruded, infrared-cooked and flaked pea seed were quite high (86%, 93% and 89% respectively) and higher or comparable to those of soybean meal (Fontainhas-Fernandes et al., 1999). Oreochromis niloticus fry fed diets containing 12, 20 and 25% peas (treatment unknown) had a higher daily gain but also a higher feed conversion ratio than the control group. Feed costs for the pea groups were lower in the starter and juvenile stage and higher in the adult stage compared to the control. Feeding peas gave a similar performance to that of soybean oilmeal and they could be a viable alternative, as a plant protein substitute, without affecting growth performance (Hill et al., 2006).

Turbot (Psetta maxima)

The protein digestibility of whole extruded peas in turbot was found to be high (93%) but the DM, starch and energy digestibilities were lower (71-78%) and it was suggested that dehulling could help improve digestibility (Burel et al., 2000).

Crustaceans

Shrimp (Litopenaeus spp.)

In juvenile blue shrimp (Litopenaeus stylirostris) fed peas at 30% in the diet, raw and processed peas were found to be very acceptable ingredients. Extrusion-cooking improved feed conversion ratio and protein efficiency ratio, and infrared-cooking enhanced feed intake, while dehulling had no effect. The ingredients also conferred differential water stability properties with the diet containing whole-extruded peas having the lowest dry matter and crude protein loss following a 1 hour immersion in water (Cruz-Suarez et al., 2001). In a similar study carried out with Litopenaeus vannamei shrimps, extrusion-dehulling and infrared-cooking resulted in the highest protein digestibility (79%) and energy digestibility (94%), though dehulling did not significantly improve digestibility. In another growth trial, where peas were included at up to 25% in the diet, there were no signiﬁcant eﬀect on survival, growth or feed efficiency values (Davis et al., 2002).

Tiger shrimp (Penaeus monodon)

In juvenile tiger shrimp (Penaeus monodon) diets, whole raw pea meal showed very good potential as a substitute protein source up to 100% of the protein from defatted soybean meal, which is equivalent to 25% of the total protein in the diet. An inclusion level of up to 42% did not manifest any adverse effects on growth, feed intake, feed conversion ratio, survival, body composition, and digestibility coefficients for DM and protein of the shrimp (Bautista-Teruel et al., 2003).